Very little is known about the mechanisms by which pathogenic mycobacteria infect the host. In addition, there is almost no information in the literature on how the bacterium regulates gene expression in the host environment. Bacterial pathogens, which maintain long-term residence within the host cells, also need a variety of strategies to help them adapt to harsh environmental conditions. Among them is the response to stress conditions, suggesting that intracellular pathogens experience a considerable amount of protein misfolding and damage within host cells. Similarly, the regulation of mycobacterial genes and coded proteins upon interaction with the host mucosa, and within macrophages are not well known. Lon protease has been shown to participate in the regulation of stress responses in Escherichia coli. In addition to the function as scavenger of damaged proteins, Lon performs important regulatory functions by degrading cellular proteins that control gene expression. The Lon protease has also been shown to be important for Brucella abortus survival in the host. More recently, Lon has been demonstrated to regulate the expression of Salmonella invasion-related genes by proteolysis of factors required for hilA (a major regulator of the pathogenicity island 1, SP11) expression. Inactivation of Ion in Salmonella typnimurium leads to a significant increase in the ability to invade intestinal -407 epithelial cells in vitro. Inactivation of Salmonella Ion was also associated with attenuation of the bacterium in human macrophages and mice. Mycobacterium avium and M. tuberculosis interact with the host mucosa (s) at the initial stages of the infection. Upon entry into the airways and intestinal tract, gene regulation needs to occur in a rapid fashion. While a number of required genes appear to be up-regulated by conditions existing in the extra-host environment, many other genes are not. Our hypothesis is that the regulation of many mycobacterial virulence genes may likely be influenced by the action of energy dependent regulatory proteases such as Lon, i.e. using protease-mediated post-translational regulation to rapidly adapt to different conditions. Recent work using subtraction hybridization, suggests that Lon is a protein required for the efficient interaction between M. avium and the intestinal mucosa . The Ion gene is present in both M. avium and M. tuberculosis genome sequence. We, therefore, propose to gain new insight in the post-translational regulation in mycobacteria by: 1- Determining whether the overproduction and/or inactivation of Ion in M. avium (as a model organism) impacts the ability to replicate in macrophages and invade mucosal epithelial cells. This work has the potential to unveil mechanisms used by mycobacteria to regulate virulence genes in the environment of the host. In case we confirm our hypothesis, future work will address the role of Ion in M. tuberculosis [unreadable] [unreadable] [unreadable]